Patent GB-899921-A (GB)
Title: Transistor pulse amplifiers
Inventor: ALLEN DENNIS WILLIAM
IPC: H03F1/30
Filed: 1959-07-17
Abstract: 899,921. Transistor circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 12, 1960 [July 17, 1959], No. 24605/59. Class 40(6). An amplifier comprises a direct current coupled transistor amplifier having the input signal applied to one base of an emitter coupled pair and with means for clamping from time to time the input voltage to a reference value and means for feed back a drift correction voltage from the output of the amplifier to the other base of the pair while the input voltage is clamped and maintaining the voltage during the period between clamping. Fig. 1 shows a direct coupled amplifier 5 connected to an input terminal 1 through a resistor R1 and a difference unit 3 and connected to the output terminal through a gate 6. A chopper 2 periodically clamps the input voltage to earth and at the same time a chopper 8 is open circuited so that the residual output voltage is fed back through a filter storage unit 9 to a difference unit so as to correct the drift. When the choppers are restored to the original condition the signal is again fed to the amplifier while an input circuit to the filter is connected to earth so that the output of the filter remains substantially constant until the next chopping cycle. In Fig. 3 the input chopper comprises a transistor V1 to the base of which negative going chopping pulses are applied, simultaneous pulses being applied through resistor R4 to compensate for the voltage drop in the transistor when it is conducting. The output of a chopper is applied to a "long tail pair" amplifier V2, V3 which feeds a direct coupled amplifier V4, V5, V6, two stages of which are coupled by a voltage dropping Zener diode Z. The output of stage V6 is coupled through a resistor R2 to an output chopper V7 which is rendered non- conducting during the chopping cycle by pulses applied through lead 10 to its base so that the drift voltage charges a capacitor C2 which is connected to the input of transistor V3 of the input circuit to cancel the drift. The output of the amplifier is connected to a capacitor V1 the right hand terminal of which is connected to earth through a transistor V8 when the input chopper is non-conducting. Accordingly during this period the capacitor charges to the output voltage. During the chopping cycle the transistor V8 is rendered non-conducting and the output of V6 returns to the reference potential so that a pulse is supplied from the right hand electrode of the capacitor to a transistor V9, this pulse being delayed by a period corresponding to one of a chopping cycle with respect to the control pulses for the input chopper. The output from transistor V9 is applied to a monostable circuit V10, V11  producing a final output at O. A rectifier MR2 prevents negative going pulses from operating the output circuit. Thus so long as an input of the desired polarity is provided at input terminal 1 pulses are produced at the output which are delayed with reference to the chopping control pulses applied to the input chopper. The circuit is suitable for use in an analogue to digital conversion system and Fig. 4 shows a modification for use where a long period may exist between "words". In this modification the time constant of the drift correcting capacitance circuit is increased and the choppers are rendered operative during the between "word" period. The output circuit is replaced by a complementary stage V14, V15  directly coupled to the amplifier output transistor V6 and having choppers V12 and V8 connected to its input and output circuits. Pulses having a repetition frequency corresponding to that of the pulses fed to the choppers of the previous embodiment are fed to the bases of transistors V12 and V8. This has the effect of converting the output signal into delayed output pulses in the same manner shown in first embodiment. It is stated that the chopper at the output of the amplifier could be arranged so that the input to the drift correcting capacitor is open circuited instead of short circuited during the chopping cycle and that the input signal may be in pulse form or may be converted into pulses by the choppers.